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push!

Must:
  1. Check for full stack

  2. Add the new item to the cells

  3. Increment height

  4. Return the modified stack 
     (define push!
       (lambda (ra-stack item)
	 (if (<= (vector-length (cells ra-stack)) ;; 1
		 (height ra-stack))
	     (error "PUSH!: attempted push onto a full stack")
	     (__________))))
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